American Mineralogist, Volume 96, pages 1433–1442, 2011 3+ The turquoise-chalcosiderite Cu(Al,Fe )6(PO4)4(OH)8·4H2O solid-solution series: A Mössbauer spectroscopy, XRD, EMPA, and FTIR study YASSIR A. ABDU,1,* SHARON K. HULL,2 MOSTAFA FAYEK,1 AND FRANK C. HAWTHORNE1 1Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 2Department of Anthropology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ABSTRACT Eight turquoise samples covering a wide range of compositions in the turquoise-chalcosiderite solid-solution series were analyzed by Mössbauer spectroscopy, X-ray diffraction (XRD), electron microprobe analysis (EMPA), and Fourier transform infrared (FTIR) spectroscopy. Two of the turquoise samples display evidence of alteration from weathering processes. The unit formulas were calculated on the basis of 24 (O,OH) anions and 11 cations using the results of EMPA, assuming all Fe as Fe3+, as confirmed by Mössbauer spectroscopy. The altered turquoise samples show deficiencies in both cations and anion groups, indicated by EMPA, but they preserve the crystal structure of turquoise, as verified by XRD. They also show large amounts of Si and Ca in their microprobe data, due to the presence of kaolinite and Ca carbonate, respectively, which are identified by FTIR spectroscopy. The isomorphous substitution of Fe3+ for Al in the turquoise structure broadens and shifts the IR bands to lower frequencies, in particular the OH-stretching bands. The Mössbauer spectra, collected at room temperature, are fitted with two generalized Fe3+ sites, using a Voigt-based quadrupole-splitting distri- bution method, which are assigned to the M3 (smaller quadrupole splitting) on the one hand and M1 and M2 octahedral sites on the other hand. The Fe3+ distribution over the M3 and M1,2 sites, calculated from the Mössbauer relative areas and EMPA, indicates that Fe3+ prefers the larger M3 octahedron in the turquoise-chalcosiderite solid-solution series. Keywords: Turquoise, chalcosiderite, alteration, Castillian Mine, Tyron Mine, Cornwall, electron microprobe analysis, FTIR, Mössbauer spectroscopy INTRODUCTION (EMPA), FTIR and Mössbauer spectroscopy, to (1) learn more Turquoise is common in the American Southwest, where about turquoise alteration and (2) study the distribution of Fe most of its value is associated with art (e.g., jewelry) and Na- between the metal sites. tive American culture. There has been renewed interest in this BACKGROUND complex mineral due to a recently developed technique that can identify the provenance regions of turquoise artifacts using the Turquoise is a hydrated copper aluminum phosphate and isotopic ratios of hydrogen and copper (Hull et al. 2008). This belongs to a group of minerals, the turquoise group, consisting approach allows archaeologists to reconstruct and investigate of at least six isostructural end-members (Table 1; Foord and pre-Colombian turquoise trade structures, and gemologists to Taggart 1998). The general formula for the turquoise group may 2+ 2+ authenticate the source of turquoise (e.g., Lone Mountain mine be written as A0–1B6(PO4)4(OH)8⋅4H2O with Cu or Fe as the 3+ 3+ in Nevada vs. Sleeping Beauty mine in Arizona). most common constituents at the A position and Al and Fe at 2+ 2+ When affected by near-surface conditions and extended the B position. However, Ca or Zn can occur at the A position exposure to sunlight and meteoric (rain) water, turquoise in some of the more rare members of the turquoise group. The weathers to chalky white clay minerals. This alteration affects range in chemical composition (i.e., different concentrations of identification of the provenance regions of archaeologically Cu, Fe, and Al) of turquoise (sensu lato) produces a wide range recovered turquoise artifacts (Hull et al. 2008), the mineral- of colors (Table 1). Foord and Taggart (1998) suggest that differ- ogical properties of turquoise, and the value of turquoise as a ences in the Cu and Fe ratio are responsible for the differences semi-precious gemstone. However, this alteration process is in color of the samples: blue turquoise has Cu at the A position poorly understood. Therefore, to improve the characterization and Al at the B position, whereas green turquoise (chalcosiderite) 3+ of turquoise provenance for archaeological and gemological largely contains Fe at the B position. purposes, it is important to understand the alteration processes The crystal structure of turquoise is triclinic, space group that affect turquoise. P1, and is illustrated in Figure 1. Different site nomenclatures In this work, we characterize turquoise samples covering have been used in many of the structural and spectroscopic a wide range of compositions in the turquoise-chalcosiderite studies of the minerals of the turquoise group, depending on series by X-ray diffraction (XRD), electron microprobe analysis the chemical compositions under study. Here we use a more general site nomenclature that avoids phrases as for example * E-mail: [email protected] “Fe3+ occupying an Al site”. There are four sites that are octahe- 0003-004X/11/0010–1433$05.00/DOI: 10.2138/am.2011.3658 1433 1434 ABDU ET AL.: THE TURQUOISE-CHALCOSIDERITE SOLID-SOLUTION SERIES TABLE 1. Minerals of the turquoise group Mineral Chemical formula Color Luster Hardness 2+ Aheylite Fe Al6(PO4)4(OH)8⋅4(H2O) pale blue, green and blue-green vitreous, dull 5–5.5 3+ Chalcosiderite CuFe 6(PO4)4(OH)8⋅4(H2O) apple green, darkgreen vitreous, glassy 4.5 “Coeruleolactite” (Ca,Cu)Al6(PO4)4(OH)8⋅4–5(H2O) milk white, light blue vitreous, waxy 5 Faustite (Zn,Cu)Al6(PO4)4(OH)8⋅4(H2O) apple green chalky, earthy, dull 5.5 Planerite Al6(PO4)2(PO3OH)2(OH)8⋅4(H2O) white, olive green, pale blue, green and blue-green vitreous, dull 5 Turquoise CuAl6(PO4)4(OH)8⋅4(H2O) pale green, blue-green, turquoise blue waxy 5–6 Note: Mineral and chemical formulas from Foord and Taggart (1998). (H2O) (OH) Al sites (i.e., M sites in our notation). However, Giuseppetti et al. T1 (1989) refined the crystal structure of an Al-bearing chalcosider- 3+ M2 M1 ite (Al = 0.54 apfu) and found that Fe is partly ordered at the Fe1 T2 site (corresponds to Al3 site in the work of Cid-Dresdner 1965). The X Al1 and Al2 sites were labeled by Giuseppetti et al. (1989) as Fe2A M3 and Fe2B, respectively. Giuseppetti et al. (1989) also reanalyzed the powder X-ray diffraction data of Cid-Dresdner and Villarroel (1972) for rashleighite and cautiously suggested similar behavior of Fe3+ in intermediate compositions of the series, where it is very difficult a b to find single crystals suitable for X-ray diffraction. 57Fe Mössbauer spectroscopy, due to its ability to probe the c local environment of the Fe nucleus regardless of the degree of sample crystallinity, can be used to verify the Fe3+ site preference in the turquoise-chalcosiderite series. To our knowledge, the work FIGURE 1. The crystal structure of turquoise; M1 and M2 octahedra are shown in yellow, M3 octahedra are shown in pink, X octahedra are done on turquoise that involves Mössbauer spectroscopy is very shown in green, and T tetrahedra are shown in blue; hydrogen bonds are scarce (Belyaev and Ievlev 1989; Sklavounos et al. 1992), and omitted for clarity. (OH) and (H2O) groups are shown as red and blue the technique has never been used to address the above issue circles, respectively. in turquoise. Other Mössbauer studies of hydrated phosphate minerals are reported by Amthauer and Rossman (1984), Da drally coordinated by O2– and (OH)− anions and two T sites that Costa et al. (2005), and De Resende et al. (2008). are tetrahedrally coordinated by O2– anions, and are occupied Fourier-transform infrared (FTIR) spectroscopy is a valuable by P. The octahedrally coordinated sites may be divided into technique in studying hydrated minerals. It is very sensitive to two groups, the M1–M3 sites that are occupied dominantly the hydroxyl (OH) group and can easily distinguish between OH by small trivalent cations, and the X site that is occupied by in the structure and H2O molecules. The technique can be used medium-sized divalent cations (primarily Cu2+, Fe2+, and Zn). to supplement X-ray diffraction and chemical analysis, as FTIR Thus, the structural formula for turquoise is X (M12M22M32)Σ=6 spectra can be easily obtained for crystalline as well as poorly (PO4)4(OH)8·4H2O. In the structure of turquoise (Fig. 1), pairs crystalline materials. Recently, Reddy et al. (2006) studied in of edge-sharing M1 and M2 octahedra are linked by sharing detail two Fe-bearing turquoise samples from Arizona (U.S.A.) corners with pairs of T tetrahedra to form chains that extend in and the Kouroudaiko mine (Senegal), with Fe2O3 contents of 1.94 the b direction. These chains are linked in the a and c directions and 3.19 wt%, respectively, by FTIR spectroscopy and assigned by sharing corners with M3 octahedra, and further linkage is the different vibrational bands for both the hydroxyl (OH) and 3– provided by X octahedra that share edges with the M1 and M2 the phosphate (PO4) groups. octahedra. The heteropolyhedral framework is strengthened by a network of hydrogen bonds involving the (OH) groups in the MATERIALS AND EXPERIMENTAL METHODS formula. Of particular importance in Figure 1 are the similarities The provenances of the turquoise samples used in this study are given in and differences exhibited by the M1–M3 octahedra. The M1 and Table 2. The samples were characterized by powder XRD using a Philips (PANalyti- cal) PW1710 automated diffractometer with CuKα radiation. The PW1710 system M2 octahedra are very similar in their general stereochemistry consists of a 4kW sealed-tube type X-ray generator in a housing with a centrally and their linkage to adjacent cation polyhedra is identical.